1. Field of Invention
This invention relates to switching power supply using a printed coil type transformer; and more particularly, to improvements in the "indirect feedback" type stabilized power supply and in an arrangement for dissipating heat therefrom.
2. Description of the Prior Art
A direct feedback type stabilized power supply, such as disclosed in Japanese Unexamined Application 1989/278,259, is operated by detecting a secondary output voltage and by feeding back the secondary output voltage to a primary control circuit. In such a power supply, a photocoupler or pulse transformer is used to isolate the output voltage.
FIG. 1 shows a conventional direct feedback type stabilized power-supply, wherein a DC input voltage Vin is turned ON and OFF by a switching element Q and then applied to a primary winding Np. The noise of the peak characteristic caused by the switching is absorbed by an input capacitor Cin disposed in the input circuit of the primary winding Np. A current flowing through the primary winding Np is represented by Ip, and a voltage generated across the primary winding Np is represented by Vp. A switching current Is is induced in a secondary winding Ns and is converted by a rectifier smoothing circuit into a direct current to supply an output voltage Vout to load R.sub.L. The rectifier smoothing circuit consists of diode D1 and capacitor C1. A switching current I.sub.B is also induced concurrently in an auxiliary winding N.sub.B and is converted by another rectifier smoothing circuit into a direct current to provide operating power to a PWM control circuit. The other rectifier smoothing circuit consists of a diode D2 and a capacitor C2. A photocoupler PC feeds back a signal, which contains the output voltage Vout, to the PWM control circuit and isolates the primary circuit from the secondary circuit. The PWM control circuit transmits an ON/OFF control signal to switching element Q to keep constant the output voltage Vout.
FIGS. 2(A)-2(E) illustrate the operation of the device of FIG. 1, wherein FIG. 2(A) shows the primary winding voltage Vp; FIG. 2(B) shows the primary winding current Ip; FIG. 2(C) shows the sum of the secondary winding current Is and the auxiliary winding current I.sub.B ; FIG. 2(D) shows the auxiliary winding current I.sub.B ; and FIG. 2(E) shows the secondary winding current Is. In FIGS. 2(D) and 2(E), the broken line and the solid line correspond to the direct feedback type stabilized power supply and the indirect feedback type stabilized power supply, respectively. When switching element Q is turned OFF, the primary winding voltage Vp is equalized to the DC input voltage Vin, and the primary winding current Ip increases in a triangular waveform manner. When switching element Q is turned ON, the primary winding voltage Vp and the primary winding current Ip are nullified so that the energy stored in the primary winding is transmitted to the secondary winding. Thus, the secondary winding current Is is similar in waveform to the auxiliary winding current I.sub.B and is gradually reduced when switching element while the switching element Q is turned OFF. Since the auxiliary winding has a lower impedance than the impedance of the secondary winding, the energy stored in the transformer when the switching element Q is turned OFF is supplied first to the auxiliary winding and then to the secondary winding.
When the secondary winding output voltage Vout is controlled using the auxiliary winding voltage V.sub.B, the auxiliary winding voltage rises more sharply than the secondary winding voltage. This is because the winding voltage V is expressed as a product of an inductance L multiplied by a rate of change of the current I, that is as follows: EQU V=L(dI/dt) (1)
Because of the differences in rising characteristics, the indirect feedback type flyback converter is less accurate in controlling the secondary winding output voltage than the direct feedback type flyback converter. The indirect feedback type flyback converter uses the auxiliary winding output voltage V.sub.B to control the secondary winding output voltage Vout, as discussed.
As disclosed in Japanese Unexamined Utility Model Application 1992/8,390, the indirect feedback type flyback converter has a large capacity and that heat produced therein is dissipated through a radiator or a box using radiation plates. In a switching power supply having a large capacity, e.g. of about 100 Watts, heat generated by the electronic parts, such as the transistors and diodes used for switching, is dissipated by conduction, and heat generated Q is turned OFF. The foregoing type of switching power supply is referred to as a flyback converter.
The direct feedback type stabilized power supply utilizes a secondary system for the transfer function and has a problem in that the control system thereof is difficult to design. An indirect feedback type stabilized power supply, such as disclosed in the Japanese Unexamined Patent Application 1985/98,870, provides a solution to the control system design problem. FIG. 3 shows a conventional indirect feedback type stabilized power supply, wherein the parts performing the same functions as in FIG. 1 are denoted by like characters and are not discussed hereat further for sake of clarity. In the conventional indirect feedback type stabilized power supply, an auxiliary winding N.sub.B is substituted for the photocoupler PC to provide a channel through which the output voltage Vout is fed back to the PWM control circuit.
The operation of the FIG. 3 device is described with reference to FIGS. 2(A)-2(E). Although in FIGS. 2(A)-2(E) the waveform showing the sum of the secondary winding current Is and the auxiliary winding current I.sub.B is the same as in the case of the direct feedback type stabilized power supply, the auxiliary winding current I.sub.B is represented by a triangular waveform which rises at the beginning of the period during which switching element Q is turned OFF and the falls sharply. On the other hand, secondary winding current Is has a waveform which increases with decreasing auxiliary winding current I.sub.B, the reaches a maximum at the instant the auxiliary winding current I.sub.B is nullified, and then, gradually falls by the transformers is dissipated by convection because of the size of the size of the transformers.
FIG. 4 shows a conventional mounted switching power supply, wherein heat dissipation channels are indicated by the arrows. In FIG. 4, a mounting base 40 formed in the shape of a flat plate from an insulating material, such as epoxy resin, is provided with a wiring pattern 42 formed from a conductive material such as copper on one side of the base 40 or on both sides thereof. An electronic part 50, such as a power diode or power transistor, is mounted on base 40. The electronic part 50 generates heat. A heat sink 60 is mounted on base 40 and is in thermal contact with electronic part 50 so that heat sink 60 dissipates the heat from the electronic part 50. A transformer 13 which comprises various windings is mounted on base 40. The transformer 13 has a core 131 disposed in the center hole of bobbin 132, around which winding 133 is wound. Winding 133 is connected to a terminal 134 which is soldered to a through hole in base 40.
FIG. 5 is a circuit diagram of the device of FIG. 4 and shows the heat dissipation channels with arrows. In FIG. 5 a DC voltage is applied from an input supply Vin via an input capacitor Cin to the primary winding n1 of the transformer 13. When a switching transistor Tr, which is connected to primary winding n1, is turned ON and OFF, a current flowing through the primary winding n1 causes a switching signal to be induced in a secondary winding n2 of the transformer 13. The switching signal from the secondary winding is rectified by a diode D and smoothed by a capacitor Cout and then supplied as an output voltage Vout to a load R.sub.L. A control signal supplied to the transistor Tr may be of an indirect feedback or of an indirect feedback. An auxiliary winding may be provided close to the primary winding n1 or the secondary winding n2 of the transformer 13.
FIG. 13 is an equivalent circuit diagram of the heat dissipation channels of a conventional device, such as shown in FIG. 4. The circuit comprises a switching transistor Q.sub.TR, a transformer Q.sub.T, and a diode Q.sub.D. These elements generate heat and are substantially separate from each other. The windings 133 of the transformer T generate heat. The heat is dissipated in two ways: first, by convection from the transformer surfaces, and second, by conduction through terminal 134. Because of high thermal resistance, the amount of heat dissipated by conduction is negligible as compared with the heat dissipated by convection. The thermal resistance is high because of the insulation and air contained between the windings and the terminal 134. Moreover, terminal 134 is of tin plated iron and hence has high thermal resistance. When core 43 meets the requirements of the Japanese Industrial Standard (JIS) EER25.5, the thermal resistance thereof is about 70.degree. C./W.
On the other hand, when transformer T is cooled by convection only, the volume thereof must be increased by a factor of 2.8 times if heat loss is doubled (see Page 280 of "Thermal Designs of Electrical Devices"). Accordingly, transformer size is increased with increased power consumption.
Also, when heat is dissipated by convection, thermal resistance varies with the placement of the parts surrounding the transformer. That is to say, the thermal resistance of a section exposed to effective air flow is reduced. On the other hand, the thermal resistance of a section cut off from the effective air flow is increased. The self cooling of a transformer by convection varies widely with the positioning of the parts thereof. Thus, power supply design and thermal design cannot be separated from each other, and the interdependence thereof makes design of such power supplies both complex and difficult.